Many gas passage ducts, such as air conditioning ducts, gas turbine inlet or exhaust ducts, and the like, are required to attenutate sound while at the same time being required to transmit gaseous flow with a minimum of back pressure. In the past, it has been customary to line the interior wall of such ducts with acoustical material, such as fiberglass, to absorb the unwanted sound. Such lined ducts are capable of efficiently attenuating higher frequency sounds, but would fail to attenutate lower frequency sounds and, hence, are often unsatisfactory. The reason for the failure to absorb low frequency sound is that for such an acoustical liner to become an efficient absorber of low frequencies, the liner must be quite thick, on the order of one-fourth the wave length of the sound. Commonly, this amounts to more than 0.75 meters of depth. Space is seldom available for such liner thickness. Additionally, a liner of such a thickness would be of significant weight and expense.
It is known that low frequency noise can be attenuated provided there is an expansion chamber. Such an expansion chamber is often called a surge tank and is added to the system in series with the lined duct section. To be effective, however, the expansion chamber must be quite large in diameter and the series connection of the two dissimilar acoustical components considerably increases its total length. Again, space is seldom available and the expense and weight can be prohibitive.